Hey guy's. Over the past couple of months I've been trying to figure out whats causing the vibration in my car. This vibration problem started at the end of March this year.

It's very noticeable after the converter locks up at 40 mph. The faster you go (no matter is the converter is locked up or if you are in another gear) the vibration gets worse. The car shakes at freeway speeds (70 mph - 80 mph).

I saw that the flex plate was warped I replaced it with and SFI approved flex plate. After it was replaced the vibration was still present.

I also bought a new set of tires - my old tires were worn out. They were also balanced properly prior to install. This didn't fix the vibration problem either.

I installed an Inland Empire 3'' drive shaft with new u-joints last summer and a BMR non-adjustable torque arm.

I also have doubts of the effectiveness of the motor mounts though because last summer the transmission mount failed. When this happened the transmission jerked up and slammed down on the cross-member every time the motor spooled up. I'm not sure if this messed up the engine mounts or not. Although the transmission mount failed last summer the car didn't have a vibration issue at all after I replaced the broken mount.

I'm not sure what else to check or look for at this point.

 

Which transmission mount did you install? Stock rubber or aftermarket polyurethane? If so, which brand?

What is your alignment? PSI in your tires? Is your car lowered or stock height?

Do you feel the vibration in the steering wheel, or more like in the shifter or under your seat?

 

I installed a stock style rubber replacement transmission mount.

I haven't had the car aligned in a while that's something that I'll look into.

I set the tire pressure to 35 psi.

My car is stock height. In addition to the suspension mods in the first positing I also have Hotchkis subframe connectors, panhard bar, and boxed lower control arms. I have these Hotchkis components on my car for over 5 years with no problems.

I feel more of the vibration in the middle of the cowl area where the motor and trans are bolted together more than anyplace else.

 

Those were the two things I was wondering. I had the same problem the op is describing and it turned out it was the drive shaft / poly trans mount.

I had changed to an aluminium drive shaft and while the shaking above 70 had calmed down quite a bit, it was still there. Later I read somewhere on this forum that the ploy trans mount throws off your pinion angle (correct me if I'm wrong hellz_wings) and a switch to the correct height trans mount would cure the vibration. I followed the advice I was told about the trans mount and now the vibrations are completely gone, I've been up to 130 and still have not felt the vibrations.

 

Yes the poly mount will give you vibrations.. Although, I switched from an Energy Suspension poly mount to a Prothane mount and vibrations were greatly reduced. I don't know how much difference it would have than rubber at this point but it's still very driveable.

Aluminum driveshaft cures high speed vibrations too..

35psi seems like alot. I usually run anywhere between 28 and 32.. Most of the time at 30psi.

Stock height meaning stock springs? If they are the original springs, you could be as low as 2" from stock (most likely around 1" lower). The old springs sag over time. Measure the height from the ground to the center of your fender. Stock height is just about 28" high (some people say 27.5", so anywhere between there is pretty much stock).

Since you installed a non-adjustable TQ arm like me (I wish I could go back and purchase an adjustable one) you cannot correct pinion angle, which could have an effect as well especially if your car is lowered.

I would go for the alignment before anything else, make sure you get CUSTOM specs, NOT the stock GM specs. Here are some recommended specs:

Camber: -0.5 to -1.0 (left and right sides equal.. more negative camber generally makes it handle better but wears out tires faster)
Caster: Get as much as you can on the passenger side, and then put the driver's side at 0.5* less
Toe-in: 3/32 (total toe is 0.09)

 

Thanks for the tips. I was wondering last night if I should have got an adjustable torque arm. I'm still running the factory springs and you are right - they have to be sagging a couple of inches. The alignment may may solve these issues too.

 

Installing a new aluminum drive shaft helped cure some of the vibration problems. My stock drive shaft was twisting itself out of shape and was visibly warped so that was part of the problem.

I'll have to check the transmission mount again and make sure that it is still tight and not cracked as well. I'll take some measurements too and see if it's in line with factory specs. It just slipped my mind that the transmission mount may be too tall or too short and that would throw off the pinion angle as well.

 

The adjustable is good for lowered vehicles, such as yours probably is now (measure center of fender to ground height to be sure where your car is at).

The alignment can cause vibrations, especially if toe is out of wack. Does the car drive straight or does the steering wheel pull to one side or the other?

 

It drives very straight and dosen't pull to one side or the other. I'll take some measurements when I get home.

I had the same trouble that you had deciding if I should get an adjustable torque arm or not when I bought mine last year. It made a big improvement in planting the rear end down for power and keeping it more stable during turns. I may end up having to get an adjustable torque arm any way.

 

Just to give an answer to this, I'm running a rubber trans mount. I switched from the Energy Suspension poly mount to a regular rubber one. It made a noticeable difference above 70 mph, next to no vibration now.

 

The ES one is crap , for sure, atleast for me and alot of people on here it was, but the prothane one seems to be much better for me.

Don't worry about adjustable unless you are very low, I am running slightly lower than stock (stock springs, but extended ball joints) and it's fine. I wanted to get adjustable but crossmember mounted to get that little extra performance out of it and reduce the chance of breaking the tranny tailshaft.

 

Thanks. I'll measure it when I get home. This situation is a Catch 22 - trying to fix the traction problem while not affecting the ride quality. Hopefully the transmission mount and double checking the ride height will give me some answers to this problem. Ha ha I've been pulling my hair out trying to figure it out.

 

lol you will get it figured out and fixed. I know the feeling, it took me a year to get rid of my 70+ vibration issue.

oh and +1 on Hellz_wings alignment specs. I noticed better handling and the outside of my tires no longer wore prematurely like they did before when I still ran stock specs. Only problem you might run into is some know-it-all arguing with you at an alignment shop, that was the only problem I had. GM should have recalled the specs so that all alignment shops could set their alignment machines to show green for the proper specs that hellz shared above. When you ask for the non GM specs it will show red on their alignment print out due to the original GM specs being so far out of whack. Whereas the GM specs will show green when they are clearly crap.